Internal combustion engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art, operate by converting heat energy of fuels to kinetic energy. In an internal combustion engine, burning of a fuel occurs in a confined space called a combustion chamber. This burning of fuel creates gases of high temperature and pressure, which expand to cause movement, for example by acting on one or more pistons. To increase efficiency of these engines it is useful to convert as much of the heat of combustion to useful work as possible, by reducing the amount of heat conducted away by the piston. Reducing heat transfer losses by way of the piston can contribute significantly to improving the operating efficiency of an internal combustion engine.
Decreasing the weight of a piston decreases its heat carrying capacity and also improves its dynamic characteristics. The weight of the piston in an internal combustion engine is typically reduced by making the piston out of a low density material such as aluminum alloys and/or steel with decreased wall thickness. While the heat of combustion is necessary to generate power, it can also damage the piston. To keep the heat from damaging the piston, the piston is often actively cooled, for example by circulating coolant through the piston and/or by splashing oil on the underside of the piston. Cooling the piston, however, increases heat extraction from the combustion chamber through the piston. Therefore, an effective mechanism for reducing the heat conducted through the piston and the heat retained by the piston, while providing for adequate cooling is required.
One method of reducing heat loss from the combustion chamber through the piston is described in U.S. Pat. No. 4,270,494 (the '494 patent), issued to Garner et al. on Jun. 2, 1981. The '494 patent describes a cross-head type piston for an internal combustion engine, which provides an insert forming a combustion bowl on the piston crown. The insert defines a central “non oil-cooled chamber” under the combustion bowl, and annular insulating air gaps under the side walls of the combustion bowl to control heat loss to circulating cooling oil. The central chamber contacts a significant portion of the combustion bowl insert and is vented to the bottom of the piston. The central chamber provides some cooling for the insert through a breathing action set up by expansion and contraction of gases within the cavity. The air gap controls heat flow primarily between the combustion gas exposed side wall of the insert and the wall of the main piston, which is exposed to a cooling oil cavity. The insert is press fitted into a recess in a main piston body and is locked in place by retaining rings. The retaining rings allow some relative movement between the combustion bowl insert and the piston body to account for thermal expansion. Cooling of the piston is provided by oil circulating through the cooling oil cavity and the venting of hot air from the central cavity.
Although the piston of the '494 patent may reduce heat extraction from the piston by providing an air gap on the side walls of the combustion bowl, it does not fully utilize an insulating air gap to reduce heat extraction from the combustion chamber. In particular, because the central cavity contacts a substantial portion of the insert, which forms the combustion bowl walls, heat extraction from the combustion bowl will be enhanced in this region. Since the bulk of the piston main body is passively cooled by the natural breathing action set up by expansion and contraction of gases within the central cavity, the piston of the '494 patent may be inadequately cooled. Also, attaching the insert to the piston main body using retention clips may increase the likelihood of the insert separating from the piston body due to inertia, during reciprocating motion of the piston.
The disclosed piston is directed to overcoming one or more of the problems set forth above.